Cell therapy using freshly isolated, ex vivo expanded or in vitro induced Tregs in models of autoimmune diseases or organ transplants have demonstrated that adoptive transfer of Tregs can restore the balance of Tregs versus effector T cells, thereby controlling autoimmunity associated with these diseases (Allan et al., (2008) Immunol. Rev. 223:391-421; Jiang et al., (2006) Expert review of clinical immunology 2:387-392; Riley et al., (2009) Immunity 30:656-665; Tang et al., (2012) Journal of molecular cell biology 4:11-21). However, the use of adoptive transfer as a therapeutic strategy presents several challenges to translation into the clinic. The number of autologous Tregs that can be isolated from peripheral blood of a human subject is limiting and extensive ex vivo expansion of the Tregs may alter their functionality and/or purity. As the isolated Tregs are polyclonal, they can exert a pan-immune suppressive function on non-target effector T cells. Importantly, the plasticity of Tregs poses a significant challenge (Bluestone et al., (2009) Nat Rev Immunol 9:811-816; Zhou et al., (2009a) Curr Opin Immunol 21:281-285), as adoptively transferred Tregs can lose Foxp3 expression and redifferentiate into Th17 cells (Koenen et al., (2008) Blood 112:2340-2352.) or pathogenic memory T cells (Zhou et al., (2009b) Nat Immunol 10:1000-1007) which raises the risk of aggravating the autoimmunity or inflammation.
A therapeutic that induces the generation of Tregs in an antigen-specific manner in situ would have advantages over adoptive Treg cell therapy. Cytotoxic T lymphocyte associated antigen-4 (CTLA-4; CD 152) is a well-established negative regulator of the T cell response, is important for the maintenance of T cell homeostasis and self-tolerance. CTLA-4 is homologous to the co-stimulatory molecule CD28 and shares the same ligands, CD80 (B7.1) and CD86 (B7.2), which are expressed on the surface of antigen presenting cells (APCs). However, differential binding of CD80/CD86 on APCs to CD28 and CTLA-4 on effector T cells leads to opposing outcomes, with CD28 triggering T cell activation and CTLA-4 causing T cell inhibition.
Because CD28 is constitutively expressed on T cells and the expression of CTLA-4 is only induced following T cell activation, peaking 2-3 days later (Jago et al., (2004) Clinical & Experimental Immunology, 136: 463-471), extensive T cell activation would have occurred prior to CTLA-4 engagement. Hence, the main role of CTLA-4 is to act as a safeguard against an excessive T cell response rather than to inhibit T cell activation. However, early engagement of CTLA-4 by its ligand and its subsequent crosslinking to the T cell receptor (TCR) can prematurely dampen TCR signaling, causing T cell inhibition and hyporesponsiveness, or anergy. This concept has been validated experimentally using a variety of methods, including the following: (i) crosslinking T cell-activating antibodies (anti-CD3/antiCD28) using an agonistic anti-CTLA-4 antibody by co-immobilization on a bead or via a secondary antibody (Blair et al., (1998) J. Immunol. 160: 12-15; Krummel and Allison, (1996) J Exp Med 183:2533-2540; Walunas et al., (1996) J. Exp. Med. 183:2541-2550); (ii) molecularly engineering a surface-linked agonistic scFv against CTLA-4 on an APC (Fife et al., (2006) J. Clin. Invest. 116 (8):2252-61; Griffin et al., (2001) J. Immunol. Methods. 248 (1-2):77-90; Griffin et al., (2000) J. Immunol. 164 (9):4433-42); and (iii) chemically crosslinking antibodies that recognize specific antigens on an APC to an agonistic anti-CTLA-4 antibody (Li et al., (2007). J. Immunol. 179 (8):5191-203; Rao et al., (2001) Clin. Immunol. 101 (2):136-45; Vasu et al., (2004) J. Immunol. 173 (4):2866-76). Restoring the balance of Tregs versus effector T cells is a promising means of treating autoimmune disease. However, cell therapy involving transfer of Tregs has certain limitations. Accordingly, therapeutics that can induce the generation of Tregs (e.g., CTLA-4) in an antigen-specific manner for the treatment of autoimmune disease are urgently required.